The present invention relates to an electro-absorption (EA) modulator integrated light source (i.e., a light emission element) having a plurality of transmission properties or characteristics being different to each other, and further relates to a light emission element module for use in an optical transmission and an optical transmitter, and also an optical transmission system using therein.
Relevant prior arts will be mentioned in relation with first to third ones, below.
Relating to the first relevant art as the light emission element, by referring to attached FIGS. 1 to 4, the structure of a semiconductor Ea modulator integrated DFB (Distributed Feedback type) laser is explained, wherein a laser is used in a light emission portion.
The FIG. 1 shows the semiconductor EA modulator integrated DFB laser having wavelength of 1.5 xcexcm, for use in an optical transmission of in transmission speed of 10 Gbit/s and 20 km in distance thereof. In this figure is shown a cross-section view of a portion of the stripes of the light emission element, for explaining the structure of the light emission element. This light emission element is formed, after forming a mask of oxidization film for a selective growth method on a n-type InP semiconductor substrate 100, with growing a lower optical separete-confinement-heterostructure 101 of n-type InGaAsP with the known selective growth method as a first growth of crystal, a strained multiple-quantum-well structure 102 composed of an undoped InGaAsP well layer and eight (8) cycles of barrier layers of undoped InGaAsP having a composition wavelength 1.15 xcexcm, and an upper optical separete-confinement-heterostructure 103 of two (2) layers of an undoped InGaAsP layer and a p-type InGaAsP layer. With using such the method of the selective growth, in the total thickness thereof, the strained multiple-quantum-well structure in an EA modulator portion 108 is formed to be thinner than that in the laser portion 109. Accordingly, an absorption wavelength of the strained multiple-quantum-well structure in the EA modulator portion comes to be smaller than that of the laser portion 109. Further, the semiconductor EA modulator integrated DFB laser shown in the FIG. 1 is manufactured, by forming a diffraction grating, a p-type InP clad layer 104, a mesa layer and a re-growth of a Fe-InP layer 105 for concealing both sides of that mesa layer, and then electrodes 107. The modulator length, i.e., the length for injecting current into a wave-guide portion of the EA modulator, is selected to be 157 xcexcm, by taking a capacity of the modulator portion and an extinction ratio thereof into the consideration for determining a band of the light emission element, and on a front end surface at the side of EA modulator is treated an antireflection coating 110, while on a terminal end surface a reflection coating.
Further, the FIG. 2 shows a light emission element module being installed with the above-mentioned light emission element thereon. The reference numeral 201 shown in the FIG. 2 indicates a chip carrier, on which the above-mentioned light emission element is mounted, and on which are formed strip lines for high frequency with a patterning technology or method, thereby building up a chip capacitor(s) and a terminal resistor(s), etc., within the light emission element module. Further, within the present light emission element module are installed or integrated a thermistor 202, an isolator 203, a lens 205, a high frequency signal relay substrate 206, a monitor PD install stem 209, and a cooling stem 208. A reference numeral 207 indicates a high frequency signal cable for electric signals.
With this optical element module, the transmission is possible on an ordinary fiber of 20 km (dispersal value: 400 ps/nm). However, the transmission is impossible on the fibers other than the ordinary one, being longer than 20 km, such as the fiber of 40 km (dispersal value: 800 ps/nm).
The reason of this lies in that the distance of optical transmission is restricted by chirping. Ordinarily in the transmission on the optical fiber, two factors, i.e., the chirping and an intensity of optical output can be mainly considered, of restricting the transmission distance. The restriction due to the intensity of optical output in the latter brings about no problem, since it can be amplified to a certain degree. The main problem here is the restriction due to the chirping in the former. The chirping means an expanse in the wavelength spectrum of light emitted from a semiconductor laser modulated. The reason or mechanism that the chirping restricts the transmission distance is as follows.
The chirping is caused by the following two (2) phenomena. First, during ON/OFF modulating in the light emission element, the chirping occurs in the wavelength due to changes in the refractive index and the absorption coefficient inside the light emission element. Second, it is a phenomenon that the chirping occurs since dispersion is generated when the light emitting from the light emission element propagates within the fiber. Accordingly, the larger the distance in the transmission distance of the fiber, the much more the chirping be caused by the latter. Further, when occurring the chirping too much, the wave-form of light signal is distorted to increase a pass penalty, thereby restricting the transmission distance.
A ratio, between the changes in refractive index and the absorption coefficient during the ON/OFF modulation of the light emission element, is an xcex1 parameter, and that is one of the causes of bringing about the chirping, and the lower the xcex1, the less the amount of the chirping during the modulation, then it can be said that the fiber is endurable against the dispersion. Therefore, the smaller the xcex1 parameter, the less the ill influence, thereby enabling to extend the transmission distance without receiving the ill influence from the dispersion.
Also, with the a parameter, there are several methods for evaluation thereof, such as, one in which a large signal is inputted into the light emission element to measure it, or other in which a small signal is inputted to measure it by a fiber-response-peak method, etc. However, in the present specification, the xcex1 parameter is defined by an evaluation value in accordance with the fiber-response-peak method, wherein the small signal is inputted to the light emission element and a dispersion compensated filer is used (F. Devaux et al., xe2x80x9cSimple Measurement of Fiber Dispersion and Chirp parameter of Intensity Modulated Light Emitterxe2x80x9d J. Lightwave Technol., vol. 11, pp. 1937-1940, December 1993). Since the xcex1 parameter is defined as a ration a change amount in refractive index to that in absorption coefficient, i.e., (change amount in refractive index)/(change amount in absorption coefficient), it varies following voltage applied to the modulator portion of the light emission element, depends upon material and MQW (multiple-Quantum-Well) structure thereof, however the light emission element comes to have almost it""s own value if it is manufactured under a certain condition of a specification, though there may be brought about a fluctuation therein a little.
The dependency (hereinafter, xe2x80x9cxcex1 curve(s)xe2x80x9d) upon voltage applied to the EA modulator having a typical xcex1 parameter of the light emission element manufactured with those relevant arts, is shown in FIG. 3 by a curve (a). In the FIG. 3, the a parameter depicted by a curve (a) indicates a value from 0.1 to 1.0, i.e., at applying voltage around (Vmodxe2x88x92Vmod)/2 when the voltage applied to the EA modulator is set at the modulation amplitude Vmod of the EA modulator and the amplitude when it is set at high level VOH. In case of this value of the xcex1, according to the evaluation of the optical fiber with the light emission element module of those related arts, it is possible to satisfy a standard, which is desired in a pass penalty when transmitting the light signal through it at a distance of 20 km (dispersal value: 400 ps/nm), for example. However, when conducting the light transmission at the distance being greater than that of the ordinary fiber, i.e., 20 km, for example, at the distance 40 km (dispersal value: 800 ps/nm), the pass penalty exceeds the standard value thereof, therefore it is difficult to apply it into a practical use thereof. Accordingly, the EA modulator integrated DFB laser shown in the relevant arts is only applicable to a system having an optical transmission system, in which the dispersion in frequency of the light due to the optical fiber is compensated for every 20 km long, namely, it can be said to be the EA modulator integrated DFB laser for a version of 20 km.
An example of the configuration of an optical transmission system wherein the light emission element module is applied is shown in FIG. 4. After an optical transmitter apparatus 401 according to the relevant arts, it is constructed with providing an optical pre-amplifier 402, the optical fibers 403 of 20 km (dispersal value: 400 ps/nm), dispersion compensation fibers 404 being provided at ever distance of about 20 km, an optical post-amplifier 405, and an optical light receiver 406.
Next, explanation will be given on the structure of the second relevant art, by referring to the FIGS. 1, 3 and 4.
This explanation is one for an example, into which is applied the semiconductor EA modulator integrated DFB laser having a wavelength band of 1.5 xcexcm, for use in optical transmission at transmission speed 10 Gbit/s and 40 km.
An aspect, differing from the first relevant art mentioned above, in particular, in the manufacturing processes or steps thereof, lies in an active layer portion of the semiconductor which is formed by the crystal growth method at first time. Explaining this by using the FIG. 1, the lower n-type InGaAsP optical separete-confinement-heterostructure 101 is laminated at thickness of 58 nm, and then the undoped strained multiple-quantum-well structure 102 and the undoped upper optical separete-confinement-heterostructure 103 at the thickness of 60 nm, with such the selective growth method. In this instance, the multiple-quantum-well structure is manufactured so that a set of wavelengths of the barrier layers InGaAsP in the strained multiple-quantum-well structure 102 is set at 1.3 xcexcm and also the number of the well-layers is seven (7). Further, the sizes of the light emission element are so designed that the injection length in the EA modulator is at 177 xcexcm, although it is at 157 xcexcm in the first relevant art, but others than those are in the condition same to that of the first relevant art. Furthermore, a static property or characteristic and a high frequency property of characteristic, after being installed into the light emission element module, are almost same to those of the light emission element module, in which the light emission element of the first relevant prior art is installed. However, the xcex1 parameter indicates a value lower that the xcex1 in the light emission element module of the first relevant art, as shown in the FIG. 3(b), and the mark or polarity of the xcex1 is changed over into a negative one when applying voltage around (VOHxe2x88x92Vmod)/2 to the EA modulator. According to the transmission evaluation of optical fiber with the light emission element module, in which is applied the light emission element shown in this relevant prior art, differing from that of the first relevant art, it is possible to satisfy the standard of the desired pass penalty value when transmitting the light signal through the ordinary fibers of 40 km (dispersal value: 800 ps/nm). This is because the chirping is decreased down in the amount thereof due to the low xcex1, when conducting ON/OFF modulating on the light emission element, as a result of this the chirping amount lies within an acceptable region thereof, even if the transmission distance in the optical fiber(s) is lengthened or elongated.
Explanation will be given on the structure or configuration of the optical transmission system with using an optical transmission apparatus, in which is installed the present light emission element module, by referring to the FIG. 4. Assuming that the optical transmission apparatus according to this relevant art is indicated by a reference numeral 401, it is constructed by comprising an optical pre-amplifier 402, optical fibers 403 having a length of 40 km (dispersal value: 800 ps/nm), an optical post-amplifier 405, and an optical receiver 406. Namely, at the distance of 40 km, the dispersion compensated fiber 404, being necessary in the case of the first relevant art, is not necessary here. On a while, in a case where an optical transmitter according to this relevant art is applied into the optical transmission system shown in the FIG. 4, into which the dispersion compensated fibers are inserted, the chirping is compensated too much because of the insertion of the dispersion compensated fibers therein. Because of this, an excessive dispersion occurs, and the pass penalty goes over the standard, therefore it is also impossible to be applied into practical use. Accordingly, the EA modulator integrated DFB laser shown in this relevant art is only applicable to a system having an optical transmission system, in which no compensation is made on the frequency dispersion of the light up to 40 km, i.e., it can be said to be the EA modulator integrated DFB laser for a version of 40 km.
Accordingly, as shown in those first and second relevant arts, the EA modulator integrated DFB lasers, each having different xcex1 parameters thereof, are manufactured, and the different systems are construct therewith. Namely, the EA modulator integrated DFB laser is used differently, depending upon the dispersal values of the optical fibers.
Further, the technology of the third relevant art will be explained as below.
The structure of the device, in which two (2) modulators are constructed in series in the DFB laser, is already known by, for example, in K. Sato et al., Tech. Digest of ECOC., (1993), WeC7, 2. xe2x80x9cA Multi-section Electroabsorption Modulator Integrated DFB Laser for Optical Pulse Generation and Modulationxe2x80x9d. In case of this construction, the two (2) modulators, i.e., a modulator 1 and a modulator 2, being formed in series to the DFB laser, are totally same to each other in the structures of modulator, including, in such as the crystal structure and the modulator length thereof, and they also have the same xcex1 parameter therewith. Accordingly, when performing the optical transmission with this light emission element, the xcex1 parameter has almost equal value in both cases when the modulator 1 is driven and when the modulator 2 is driven, therefore the transmission property or characteristic comes to be the same one. Further, according to the mode in using the light emission element which is described in this relevant art, the modulator 1 of the two of them is applied for generating a RF pulse while the modulator 2 as an encoder. This is the light emission element for the purpose of using in a so-called time-sharing multiple optical transmission, as one of the optical transmission methods.
Therefore, according to the relevant arts mentioned above, the EA modulator integrated DFB laser must be designed depending upon each different system, in different optical transmission pass or passage.
In this manner, the fact that two or more kinds of the light emission elements and the light emission element modules must be manufactured for each system (for example, for use in transmission of 20 km, and for use in transmission of 40 km) brings about an important demerit from a view point of cost reduction that will be required in coming future. Also, from a view point on the side of using the light emission element or the light emission element module, the fact that the light emission element or the light emission element module has no compatibility between the systems causes a disadvantage in the use thereof, and it prevents from cost reduction and simplifying or shortening of the processes therefor.
For example, in a case when designing an optical transmission system from a site A in a certain city to a site B in another city, the dispersion compensated fiber(s) is/are introduced or inserted at an appropriate position(s), by taking the dispersion in the optical fiber of the distance between the side A and the side B and the a parameter thereof into the consideration, thereby to design the dispersion in the total transmission pass. However, if assuming that after that an destination in communication from the site A is changed to a site C in a new city located between the cities A and B, since the transmission distance is changed, therefore the dispersal value of the optical fiber must be changed. In this instance, since the value xcex1 of the light source of the optical transmitter is fixed according to the above relevant arts, there occurs a necessity that the dispersion compensated fiber(s) is/are build up at the optical receiver side of the site C of the new city, so as to adjust the dispersal value in the optical fibers of the optical transmission system, as a whole.
Those problems are caused by the facts that each EA modulator integrated FDB laser has only a certain one xcex1 parameter inherent and that it is no-changeable nor invariable.
An object, according to the present invention, is to provide a semiconductor electro-absorption optical modulator integrated light emission element, a light emission element module and an optical transmission system, wherein plural kinds of optical transmission systems can be structured by one light emission element, thereby obtaining simplification or shortening in the design time and in the manufacture processes thereof, as well as reduction of the cost thereof.
According to the present invention, for dissolving the above-mentioned problems, each the modulator integrated light emission element has a plurality of the a parameter characteristics.
Therefore, in a case where the light emission element is constructed with a first modulator and a second modulator, each having the respective characteristic, the first modulator 1 is used when transmitting the light from a cite A to a cite B, and then the second modulator 2 is used when the destination in communication from the cite A is changed to a cite C located between the cites A and B. In this manner, with the provision of only one of the light emission element or the light emission element module, it is possible to cope with two (2) systems.